Stratification in a Tranquil Sea

Bright talus winds downslope through crags and crannies in the banded scarps exposed in the east wall of Dionysius crater. Horizontal lineations result from differential mass wasting of stratified rock in Mare Tranquilitatis; LROC NAC M137434784; east is up; image width 450 meters [NASA/GSFC/Arizona State University].

Dionysius crater (17.297°E, 2.766°N) is situated on the western edge of Mare Tranquillitatis (the Sea of Tranquility) and excavates both highlands (bright, high reflectance) and mare (dark, low reflectance) materials. Dark banded layers of mare peek out of the eastern wall, where mare material was disturbed by the impact that formed Dionysius crater. Bright talus trails wind downslope through crags and crannies in the dark mare scarps.

Looking closely, the mare appears banded or striated, indicating a non-uniform material. In general, mare are thought to form from large volumes of fluid lavas, much like the Columbia River Basalts in the Pacific Northwest of North America. The stratifications in the lunar mare may represent a series of lava flows in the region.

Blocky overhangs indicate areas more resistant to mass wasting and are interpreted as more coherent basaltic (mare) materials. The thinner, more finely grained layers might represent boundaries between individual lava flows or they may indicate changes in physical properties within a single flow unit. Some of the fine grained layers may even consist of paleoregoliths, ancient regolith surfaces exposed to the vacuum of space in between volcanic eruptions.

In any case, craters such as Dionysius provide windows into the subsurface structure of the lunar mare. With further study, the total thickness of the mare, as well as the structure and flow mechanics of individual mare flows may be intuited from this and other mare exposures in the walls of impact craters.